A great number of processes have heretofore been proposed for separating enriched oxygen from air through pressure swing adsorption by using a zeolite adsorbent which is capable of selectively adsorbing nitrogen.
One type of process is disclosed, for example, in Japanese Patent Publication Nos. 53-46800 (46800/1978) and 57-50722 (50722/1982) and Japanese Patent Public Disclosure No. 58-135106 (135106/1983). In the processes disclosed therein, adsorption is effected at a pressure of 2 kg/cm.sup.2 G or more, while evacuation is effected at a pressure of 200 Torr or less, and after pressurization has been effected with the product gas, the feed end of the column and the effluent end of the column after adsorption has been completed are connected together for pressure equalization, thereby advantageously realizing a high rate of recovery of oxygen and an increased amount of oxygen produced per unit amount of adsorbent (hereinafter referred to as "specific product"). On the other hand, such processes are disadvantageous that they require a high power consumption.
In Japanese Patent Publications Nos. 42-26164 (26164/1967), 51-40549 (40549/1976) and 51-40550 (40550/1976), the supply of the feed gas is suspended during the adsorption at a point where an adsorption area which has not yet been contaminated by an impurity is left at the downstream side of the adsorbent and, while cocurrent-flow depressurization is being carried out, the break-through front of impurity adsorption advances to the upper portion of the adsorption column where no impurity is absorbed, thereby taking out a product gas. Part of the product gas is introduced into a column in which regeneration of the adsorbent has already been completed, from its effluent end, thereby using the product gas for both pressurization and purging.
There are many variations in processes related to the present invention. However, since these utilize adsorption at a pressure (3 kg/cm.sup.2 G) higher than atmospheric pressure and purging at atmospheric pressure, they suffer from a low recovery rate and a low specific product. In addition, it is typically necessary to make frequent fine adjustment of the flow rate of the product gas introduced for pressure equalization and purging.
A group of recent applications, i.e., Japanese Patent Public Disclosure Nos. 60-161308 (161308/1985), 60-161309 (161309/1985), 61-133114 (133114/1986) and 61-133115 (133115/1986), utilize evacuation to lower the adsorption pressure and improve the specific power consumption and the rate of recovery of oxygen, but have not yet overcome all of the above-described disadvantages. In other words, these applications are basically analogous processes, that is, the supply of the feed gas is suspended during the adsorption step of a point where a portion of the adsorption area at the downstream side remains uncontaminated. As a necessary consequence, the specific product is reduced and, at the same time, it is disadvantageously necessary to effect complicated and time-consuming adjustment thereafter for lowering the pressure utilized for pressure equalization and purging.
Japanese Patent Public Disclosure No. 60-180903 (180903/1985) discloses a process which includes adsorption, evacuation and pressurization by a product gas. However, it is stated that, if purging by a product gas is not used in combination, the specific product lowers and the specific power consumption rises.
Utilization of enriched oxygen gas produced by pressure swing adsorption has recently been increasing in a great number of fields. However, the concentration of oxygen which can stably be produced by these known processes is 93% by volume, and the concentration of nitrogen as an impurity gas therein is about 2% to 3% by volume.
Accordingly, if the nitrogen concentration in the enriched oxygen gas is minimized, it is anticipated that the enriched oxygen produced by pressure swing adsorption will be used in a wider range of application.
A process for producing an enriched oxygen gas having a lowered nitrogen content is disclosed in Japanese Patent Public Disclosure No. 48-83078 (83078/1973). It is stated, however, that enriched oxygen cannot be obtained by a three-column system, and one example is given in which the nitrogen content was lowered from 0.27% to 160 ppm in a four-column system. However, since the specific product is considerably low, this process is considered economically impractical. Thus, there has heretofore been no oxygen enriching technique which enables production of an oxygen gas having a lowered nitrogen content while being satisfactory in terms of specific power consumption, specific product and rate of recovery.